In order to better appreciate the invention, it is helpful to present a known communication procedure and its disadvantages.
FIG. 1 shows the communication diagram of a central unit C of such a system along with the communication diagrams of the badges B1, B2, B3 dialoging with this unit.
After a pre-defined triggering action, for example a user approaching a handle of the vehicle, the central unit C of the access system switches to transmit mode and sends a so-called wake-up (WU) message.
If a user carrying a badge B1 is near the vehicle, the badge detects this wake-up message and switches to listening mode.
The central unit then emits an coded identification message CH, then chooses the badge with which it will communicate to avoid several badges simultaneously sending a response. This step of the communication protocol comprises two phases generally called the anti-collision phase ANTI and the selection phase SEL.
During the anti-collision phase ANTI, badges present around the vehicle emit a message indicating their presence. In the example shown in FIG. 1, badges B1, B3 located near the vehicle each send a pulse A1, A3 indicating their presence.
During the selection phase SEL, the central unit selects a badge by sending a pulse SEL1 characteristic of the badge B1 it has chosen. The other badge B3 then puts itself in standby mode.
The central unit then emits synchronization pulses S1, S2, S3 at regular time intervals T. On reception of each synchronization pulse, the selected badge B1 emits a coded response pulse IR1 which is a part of the response message REP. The synchronization pulses S1, S2, S3 enable the processing time of the badge to be measured. This time measurement, usually called the anti-pirate stage, enables the unlocking to be inhibited when malevolent people equipped with pirate transmit/receive units attempt to steal the vehicle. In effect, when pirate transmit/receive units are interposed between the badge and the vehicle, the central unit receives the pulse of the response message IR1 after a time longer than the processing time of the badge T owing to the delay introduced by the processing time of the pirate units.
However, the security of a communication procedure of this type is insufficient, since we could imagine that the pirate units mask the delay that they introduce by eliminating the anti-collision ANTI and selection SEL phases. In particular, pirates can trigger the communication procedure, and, after amplification, send to the badge the wake-up WU and identification CH messages. Next they interrupt the communication with the central unit and generate synchronization pulses. They receive the response pulses generated by the badge and return them to the central unit, at the right moment. Consequently, the central unit cannot detect the presence of the pirate units, and the security of the vehicle access system is compromised.